Unit pixel of image sensor and photo detector thereof

ABSTRACT

A unit pixel of an image sensor and a photo detector are disclosed. The photo detector can include: a substrate in which a V-shaped groove having a predetermined angle is formed; a light-absorbing part formed in a floated structure above the V-shaped groove and to which light is incident; an oxide film formed between the light-absorbing part and the V-shaped groove and in which tunneling occurs; a source formed adjacent to the oxide film on a slope of one side of the V-shaped groove and separated from the light-absorbing part by the oxide film; a drain formed adjacent to the oxide film on a slope of the other side of the V-shaped groove and separated from the light-absorbing part by the oxide film; and a channel interposed between the source and the drain along the V-shaped groove to form flow of an electric current between the source and the drain.

BACKGROUND

1. Field of the Invention

Embodiments of the present invention relate to a unit pixel of an imagesensor and a photo detector of the unit pixel.

2. Description of the Related Art

An image sensor is a sensor that transforms an optical signal to anelectrical image signal. When light is irradiated to a light-absorbingpart inside a unit pixel of an image sensor chip, the image sensordetects the light incident at each unit pixel and the amount of thelight and transforms an optical signal to an electrical signal and thentransfers the electrical signal to analog and digital circuits forforming an image.

The conventional image sensors can be classified into CCD (ChargeCoupled Device) types and CMOS (Complementary Metal Oxide Semiconductor)types, based on their structures and operation principles. The CMOS typeimage sensors are commonly referred to as CIS (CMOS Image Sensor).

In the CCD type image sensor, groups of signal electrons generated atthe pixels by the light are transmitted to an output unit by a pulseapplied to a gate, transformed to voltages of the output unit, and sentout one by one.

In the CMOS type image sensor, the signal electrons and holes that aregenerated at the pixels by the light are transformed to voltages insidethe pixels. These voltages are connected to a signal processor,including a row decoder and a column decoder, and sent out of the pixelsby a switching operation according to a clock frequency.

The image sensor can be either an APS (Active Pixel Sensor) or a PPS(Passive Pixel Sensor), according to the presence of an amplifier in theunit pixel.

The PPS is a passive device that does not encompass a signalamplification function inside the pixel, and outputs the electriccurrent of the device to the outside to transform the electric currentto a voltage outside the pixel. On the other hand, the APS is an activedevice that encompasses a signal amplification function inside thepixel.

The PPS is mostly constituted with one photo diode and one selecttransistor, and thus not only can have a greater aperture ratio than theAPS, which requires 3-5 MOS transistors for the same sized pixel, butalso can raise a fill factor related to a light-absorbing efficiency.

However, since the intensity of photoelectric current of the photo diodeis not great and an optical signal is transformed to electric currentthat is vulnerable to an outside environment for use in signalprocessing, the PPS has a problem of fixed pattern noise (FPN).

Therefore, for the same size pixel, the APS can provide an image signalthat has relatively less noise than the PPS, despite the smaller size ofthe light-absorbing part than the PPS, since a multiple number oftransistors are present in the unit pixel.

One electron-hole pair (EHP) is generated for one photon that isincident at a unit pixel light-absorbing part of an image sensor, andthe generated electrons and holes are accumulated in a photo diode,which is a light-absorbing part.

The maximum accumulation electrostatic capacity of a photo diode isproportional to the area of photo detection of the photo diode.Particularly, in the case of CMOS type image sensor, the area in whichthe accompanying transistors are arranged is larger than that of the CCDtype image sensor, and thus increasing the area of the light-absorbingpart is physically limited. Moreover, the photo diode, which is commonlyused as the light-absorbing part of an image sensor, has relativelysmall electrostatic capacity and thus is easily saturated, and it isdifficult to segment the signals generated by the light-absorbing part.

Therefore, the unit pixels of the CMOS image sensor require a relativelylong photoelectric charge accumulation time in order to generate aminimum electric charge for signal processing through the limited photodetection area. Accordingly, it is not easy to manufacture ahigh-density/high-speed frame image sensor by using the unit pixelshaving this kind of light-absorbing part.

The band gap of a silicon semiconductor is 1.12 eV, and a photo detectormade of a silicon semiconductor can detect light energy in wavelengthsof 350 nm to 1150 nm. Here, since the light has different inherentenergy per wavelength and has different depth of penetration when thelight penetrates silicon, which is solid, the photoelectric efficiencyfor each wavelength is also different at the photo detector. In order todetect the wavelengths of visible rays (400-700 nm), the image sensorforms an interface of P-N junction so that a green ray, which commonlyhas energy in the wavelength of 550 nm, can be better detected.Therefore, in the image sensor having this structure, photoelectricefficiencies for a short wavelength, such as blue color, and a longwavelength of a near infrared ray are deteriorated, or the opticalsignal is transformed to a noise.

Prior arts related to an image sensor and a unit pixel of an imagesensor include U.S. Publication Number 2004/0217262 (“UNIT PIXEL IN CMOSIMAGE SENSOR WITH HIGH SENSITIVITY”), U.S. Publication Number2009/0032852 (“CMOS IMAGE SENSOR”) and U.S. Publication Number2010/0073538 (“IMAGE SENSOR”).

U.S. Publication Number 2004/0217262 discloses an image sensor thatincludes one photo diode and four transistors of a transfer transistor,a reset transistor, a drive transistor and a selection transistor andthat inhibits the drive transistor and the selection transistor frombeing affected by leakage of a power supply voltage (VDD) by separatingan active area in which the drive transistor and the selectiontransistor are formed from an active area in which the reset transistoris formed.

However, since U.S. Publication Number 2004/0217262 integrates the photodiode and the four transistors in a limited area, it is difficult toprovide an area of the photo diode for generating a sufficient quantityof electric charge for signal processing.

U.S. Publication Number 2009/0032852 discloses an image sensor that canacquire a wide dynamic range without the loss of sensitivity, byallowing a pixel constituting a CMOS image sensor to have a plurality offloating diffusion regions.

The CMOS image sensor of U.S. Publication Number 2009/0032852 acquires afinal image by acquiring and synthesizing a signal, of which thesensitivity is low but the dynamic range for the brightness is wide, anda signal, of which the dynamic range for the brightness is narrow butthe sensitivity is high, in a separate floating diffusion region.

However, since the above CMOS image sensor acquires the high-sensitivitysignal and the wide dynamic range signal using the respective separatefloating diffusion regions and their related transistors, it isdifficult to provide a sufficient region for a photo detector.

U.S. Publication Number 2010/0073538 discloses an image sensor having ahigh photoconductivity. However, the image sensor of U.S. PublicationNumber 2010/0073538 forms an additional film layer over a PN junction inorder to increase the photoconductivity of a PN junction diode, and thusrequires an additional manufacturing process.

SUMMARY

Contrived to solve the above problems, embodiments of the presentinvention provide a unit pixel of a high-sensitivity/high-performanceimage sensor and a photo detector of the unit pixel that can output agreat photoelectric current with a small quantity of light, realize ahigh-speed frame operation in an environment of low level ofillumination, and record a video ranging from low to high levels ofillumination in a same screen.

An aspect of the present invention features a photo detector configuredto absorb light in a unit pixel of an image sensor transforming theabsorbed light to an electrical signal, which can include: a substratein which a V-shaped groove having a predetermined angle is formed; alight-absorbing part formed in a floated structure above the V-shapedgroove, light being incident to the light-absorbing part; an oxide filmformed in between the light-absorbing part and the V-shaped groove,tunneling occurring in the oxide film; a source formed adjacent to theoxide film on a slope of one side of the V-shaped groove and separatedfrom the light-absorbing part by the oxide film; a drain formed adjacentto the oxide film on a slope of the other side of the V-shaped grooveand separated from the light-absorbing part by the oxide film; and achannel interposed between the source and the drain along the V-shapedgroove and configured to form flow of an electric current between thesource and the drain. The light-absorbing part can be doped with firsttype impurities, and the source and the drain can be doped with secondtype impurities. The light-absorbing part can be insulated from thesource and the drain by the oxide film. Electron-hole pairs aregenerated in the light-absorbing part by the light incident to thelight-absorbing part, and tunneling can occur in between at least one ofthe source and drain and the light-absorbing part by an electric fieldconcentrated in the oxide film, and electrons of the electron-hole pairscan be moved to at least one of the source and the drain from thelight-absorbing part by the tunneling, and the flow of the electriccurrent of the channel can be controlled by a change in the quantity ofelectric charge of the light-absorbing part caused by the moving of theelectrons.

The substrate can be a {100} type silicon substrate, and the V-shapedgroove can be formed on the substrate by anisotropic etching.

The channel can be formed in a state immediately before pinch-off byadjusting a W/L ratio, which is a ratio between a width (W) and a length(L) of the channel.

The source and the drain can be formed by doping the second typeimpurities in a body, and the body can be floated.

A threshold voltage of the photo detector can be changed due to thetunneling occurred in the oxide film.

The photo detector can also include a light-blocking layer formed on asurface other than the light-absorbing part and configured to blocktransmission of light in areas other than the light-absorbing part.

Another aspect of the present invention features a unit pixel of animage sensor configured to transform absorbed light to an electricalsignal, which can include: a photo detector configured to cause anelectric current to flow using a change in the quantity of electriccharge caused by incident light; and a select device configured tooutput the electric current generated by the photo detector to a unitpixel output terminal. The photo detector can include: a substrate inwhich a V-shaped groove having a predetermined angle is formed; alight-absorbing part formed in a floated structure above the V-shapedgroove, light being incident to the light-absorbing part; an oxide filmformed in between the light-absorbing part and the V-shaped groove,tunneling occurring in the oxide film; a source formed adjacent to theoxide film on a slope of one side of the V-shaped groove and separatedfrom the light-absorbing part by the oxide film; a drain formed adjacentto the oxide film on a slope of the other side of the V-shaped grooveand separated from the light-absorbing part by the oxide film; and achannel interposed between the source and the drain along the V-shapedgroove and configured to form flow of an electric current between thesource and the drain. The select device can include: a drain beingconnected with the source of the photo detector; a source being accessedto the unit pixel output terminal; and a gate configured to receive acontrol signal from an outside, and a switching operation can beperformed based on the control signal. The light-absorbing part can bedoped with first type impurities, and the source and the drain are dopedwith second type impurities. The light-absorbing part can be insulatedfrom the source and the drain by the oxide film. Electron-hole pairs canbe generated in the light-absorbing part by the light incident to thelight-absorbing part, and tunneling can occur in between at least one ofthe source and drain and the light-absorbing part by an electric fieldconcentrated in the oxide film, and electrons of the electron-hole pairscan be moved to at least one of the source and the drain from thelight-absorbing part by the tunneling, and the flow of the electriccurrent of the channel can be controlled by a change in the quantity ofelectric charge of the light-absorbing part caused by the moving of theelectrons.

The substrate can be a {100} type silicon substrate, and the V-shapedgroove can be formed on the substrate by anisotropic etching.

The source of the photo detector and the drain of the select device canbe formed in a same active area.

The channel can be formed in a state immediately before pinch-off byadjusting a W/L ratio, which is a ratio between a width (W) and a length(L) of the channel.

Another aspect of the present invention features a photo detectorconfigured to absorb light in a unit pixel of an image sensortransforming the absorbed light to an electrical signal, which caninclude: a substrate in which a U-shaped groove having a predeterminedangle is formed; a light-absorbing part formed in a floated structureabove the U-shaped groove, light being incident to the light-absorbingpart; an oxide film formed in between the light-absorbing part and theU-shaped groove, tunneling occurring in the oxide film; a source formedadjacent to the oxide film on a slope of one side of the U-shaped grooveand separated from the light-absorbing part by the oxide film; a drainformed adjacent to the oxide film on a slope of the other side of theU-shaped groove and separated from the light-absorbing part by the oxidefilm; and a channel interposed between the source and the drain in alower area of the U-shaped groove and configured to form flow of anelectric current between the source and the drain. The light-absorbingpart can be doped with first type impurities, and the source and thedrain are doped with second type impurities. The light-absorbing partcan be insulated from the source and the drain by the oxide film.Electron-hole pairs can be generated in the light-absorbing part by thelight incident to the light-absorbing part, and tunneling can occur inbetween at least one of the source and drain and the light-absorbingpart by an electric field concentrated in the oxide film, and electronsof the electron-hole pairs can be moved to at least one of the sourceand the drain from the light-absorbing part by the tunneling, and theflow of the electric current of the channel can be controlled by achange in the quantity of electric charge of the light-absorbing partcaused by the moving of the electrons.

The substrate can be a {100} type silicon substrate, and the U-shapedgroove can be formed on the substrate by anisotropic etching to have aslope with a predetermined depth.

The channel can be formed in a state immediately before pinch-off byadjusting a W/L ratio, which is a ratio between a width (W) and a length(L) of the channel.

The source and the drain can be formed by doping the second typeimpurities in a body, and the body can be floated.

A threshold voltage of the photo detector can be changed due to thetunneling occurred in the oxide film.

The photo detector can also include a light-blocking layer formed on asurface other than the light-absorbing part and configured to blocktransmission of light in areas other than the light-absorbing part.

Another aspect of the present invention features a unit pixel of animage sensor configured to transform absorbed light to an electricalsignal, which can include: a photo detector configured to cause anelectric current to flow using a change in the quantity of electriccharge caused by incident light; and a select device configured tooutput the electric current generated by the photo detector to a unitpixel output terminal. The photo detector can include: a substrate inwhich a U-shaped groove having a predetermined angle is formed; alight-absorbing part formed in a floated structure above the U-shapedgroove, light being incident to the light-absorbing part; an oxide filmformed in between the light-absorbing part and the U-shaped groove,tunneling occurring in the oxide film; a source formed adjacent to theoxide film on a slope of one side of the U-shaped groove and separatedfrom the light-absorbing part by the oxide film; a drain formed adjacentto the oxide film on a slope of the other side of the U-shaped grooveand separated from the light-absorbing part by the oxide film; and achannel interposed between the source and the drain in a lower area ofthe U-shaped groove and configured to form flow of an electric currentbetween the source and the drain. The select device can include: a drainbeing connected with the source of the photo detector; a source beingaccessed to the unit pixel output terminal; and a gate configured toreceive a control signal from an outside, and a switching operation isperformed based on the control signal. The light-absorbing part can bedoped with first type impurities, and the source and the drain can bedoped with second type impurities. The light-absorbing part can beinsulated from the source and the drain by the oxide film. Electron-holepairs can be generated in the light-absorbing part by the light incidentto the light-absorbing part, and tunneling can occur in between at leastone of the source and drain and the light-absorbing part by an electricfield concentrated in the oxide film, and electrons of the electron-holepairs can be moved to at least one of the source and the drain from thelight-absorbing part by the tunneling, and the flow of the electriccurrent of the channel can be controlled by a change in the quantity ofelectric charge of the light-absorbing part caused by the moving of theelectrons.

The source of the photo detector and the drain of the select device canbe formed in a same active area.

The substrate can be a {100} type silicon substrate, and the U-shapedgroove can be formed on the substrate by anisotropic etching.

The channel can be formed in a state immediately before pinch-off byadjusting a W/L ratio, which is a ratio between a width (W) and a length(L) of the channel.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a tunnel junction photo detector inaccordance with an embodiment of the present invention.

FIG. 2 is another perspective view of the tunnel junction photo detectorin accordance with an embodiment of the present invention.

FIG. 3 is a cross-sectional view of the tunnel junction photo detectorin accordance with an embodiment of the present invention.

FIG. 4 is a cross-sectional view for illustrating the forming of achannel of the tunnel junction photo detector in accordance with anembodiment of the present invention.

FIG. 5 is a cross-sectional view for illustrating a light-blockingmethod of the tunnel junction photo detector in accordance with anembodiment of the present invention.

FIG. 6 is a cross-sectional view for illustrating an incident angle oflight of the tunnel junction photo detector in accordance with anembodiment of the present invention.

FIG. 7 is a circuit schematic of a unit pixel using the tunnel junctionphoto detector in accordance with an embodiment of the presentinvention.

FIG. 8 is a cross-sectional view of the unit pixel using the tunneljunction photo detector in accordance with an embodiment of the presentinvention.

FIG. 9 is a perspective view of a tunnel junction photo detector havinga V-shaped light-absorbing part in accordance with another embodiment ofthe present invention.

FIG. 10 is a cross-sectional view for illustrating an incident angle oflight of the tunnel junction photo detector having the V-shapedlight-absorbing part in accordance with another embodiment of thepresent invention.

FIG. 11 is a diagram for illustrating the V-shaped light-absorbing partin accordance with another embodiment of the present invention.

FIG. 12 is a cross-sectional view of another tunnel junction photodetector having the V-shaped light-absorbing part in accordance withanother embodiment of the present invention.

FIG. 13 is a perspective view of a tunnel junction photo detector havinga U-shaped light-absorbing part in accordance with yet anotherembodiment of the present invention.

FIG. 14 is a cross-sectional view of the tunnel junction photo detectorhaving the U-shaped light-absorbing part in accordance with yet anotherembodiment of the present invention.

DETAILED DESCRIPTION

Since there can be a variety of permutations and embodiments of thepresent invention, certain embodiments will be illustrated and describedwith reference to the accompanying drawings. This, however, is by nomeans to restrict the present invention to certain embodiments, andshall be construed as including all permutations, equivalents andsubstitutes covered by the ideas and scope of the present invention.

Throughout the description of the present invention, when describing acertain technology is determined to evade the point of the presentinvention, the pertinent detailed description will be omitted. Termssuch as “first” and “second” can be used in describing various elements,but the above elements shall not be restricted to the above terms. Theabove terms are used only to distinguish one element from the other.

When one element is described as being “connected” or “accessed” toanother element, it shall be construed as being connected or accessed tothe other element directly but also as possibly having another elementin between. On the other hand, if one element is described as being“directly connected” or “directly accessed” to another element, it shallbe construed that there is no other element in between.

Hereinafter, certain embodiments of the present invention will bedescribed with reference to the accompanying drawings.

FIG. 1 is a perspective view of a tunnel junction photo detector inaccordance with a first embodiment of the present invention. Asillustrated in FIG. 1, a photo detector of a unit pixel is realizedusing a tunnel junction instead of the conventional photo diode. Here, atunnel junction device, in which a thin insulation layer is joined inbetween two conductors or semiconductors, refers to a device thatoperates using a tunneling effect generated in the insulation layer. Forreference, the tunneling effect is a quantum mechanical phenomenon inwhich a particle passes through an area having a greater potentialenergy than its inherent dynamic energy under a strong electric field.

In an embodiment of the present invention, the photo detector of a unitpixel can be generated using said tunnel junction device, and the“tunnel junction photo detector” in the description and claims of thepresent invention refers to a photo detector realized using said tunneljunction device. The tunnel junction photo detector can be realizedusing various kinds of structures, for example, the general n-MOSFET orp-MOSFET structure. Also, in addition to MOSFET, the unit pixel can berealized using an electronic device having a structure that can providea tunneling effect, for example, JFET, HEMT, etc.

In FIG. 1, a tunnel junction photo detector 100 is realized in an NMOSstructure. The tunnel junction photo detector 100 is formed on a p-typesubstrate 110 and includes an N+ diffusion layer 120 corresponding to asource and an N+ diffusion layer 130 corresponding to a drain in ageneral NMOS electronic device. Hereinafter, the N+ diffusion layers120, 130 will be referred to as the “source” and “drain” in the tunneljunction photo detector, respectively.

Formed in between the source 120 and the drain 130 is a thin oxide film140, and formed above the oxide film 140 is a poly-silicon 150, in whicha p-type impurity is doped, corresponding to the gate in the NMOSstructure. Here, in order to facilitate the tunneling phenomenon, it ispreferable that the oxide film 140 is formed in the thickness of 10 nmor less, for example, 2 nm, 5 nm, 7 nm, etc.

Unlike a gate in a general NMOS electronic device, the poly-silicon 150is formed in a floated structure. In addition, the poly-silicon 150 doesnot form a silicide layer above the poly-silicon 150 and operates as anarea that absorbs light. If a silicide layer is formed over thepoly-silicon 150, metallic impurities make it difficult forelectron-hole pairs to be formed by the light and for the light topermeate into the poly-silicon 150 because the incident light isreflected.

Hereinafter, an area of the poly-silicon 150 of the tunnel junctionphoto detector 100 in the description and claims of the presentinvention will be referred to as a “light-absorbing part.”

Formed above the source 120 and the drain 130 are metal contacts 121,131 that are respectively connected with outside nodes. The metalcontact 121 of the source 120 is connected with an outside through ametal line 122, and the metal contact 131 of the drain 130 is likewiseconnected with an outside through a metal line 132.

The tunnel junction photo detector 100 is formed in a structure in whichthe p-type substrate 110 is floated, unlike the general NMOS electronicdevice. Accordingly, the tunnel junction photo detector 100 is differentin structure from the general NMOS electronic device in that only thesource 120 and the drain 130 are connected with the outside nodes.

Moreover, the tunnel junction photo detector 100 can be formedsymmetrically. Accordingly, it is possible that the source 120 and thedrain 130 are substituted with each other.

An upper part of the photo detector 100 excluding an upper surface ofthe light-absorbing part 150 has a light-blocking layer 180 formedthereon. Referring to FIG. 5, the light-blocking layer 180 blocks thelight from being absorbed in areas other than the light-absorbing part150, by being formed on an upper part of the tunnel junction photodetector 100 excluding the upper surface of the light-absorbing part150. This is to efficiently tunnel photoelectric charges of thelight-absorbing part 150. Moreover, this is to inhibit parasiticelectric charges from being generated by the absorption of light inareas other than the light-absorbing part 150 as well as to obtaincontrolled photoelectric current. The light-blocking layer 180 can beformed through a silicide process and can be prevented from being formedover the light-absorbing part 150 through the use of a mask.

FIG. 2 illustrates a photo detector having a micro lens. In FIG. 2, amicro lens 170 converges light incident to the photo detector 100. In acommon image sensor, the light is incident at the image sensor throughan optical lens (not shown). The light having passed the optical lensarrives at the micro lens 170 located above the photo detector 100. Themicro lens 170 converges the light incident the light incident at afront surface of the unit pixel and allows the incident light to enteran upper surface 151 of the light-absorbing part 150. Here, the uppersurface 151 of the light-absorbing part 150 can be directly exposed, ora passivation layer, through which light can readily permeate, can beformed in between the light-absorbing part 150 and the air. The microlens 170 is arranged above the light-absorbing part 150 where thelight-blocking layer 180 is not formed in such a way that the light isconverged.

An electric field is formed between the source 120 and drain 130 and thelight-absorbing part 150 by the incident light, and a channel 160 isformed between the source 120 and the drain 130. Specifically,electron-hole pairs are generated by the light incident at thelight-absorbing part 150, and electrons of the generated electron-holepairs are moved to the source 120 or the drain 130 from thelight-absorbing part 150 by a tunneling effect. Due to the loss of theelectrons, the quantity of electric charge in the holes of thelight-absorbing part 150 becomes relatively increased. Accordingly,unlike a common NMOS device, the channel 160 is formed and electriccurrent becomes to flow between the source 120 and the drain 130 due tothe effect of threshold voltage modulation caused by a change in thequantity of electric charge of the light-absorbing part 150 formed witha doped floating gate.

Meanwhile, the tunnel junction photo detector 100 can be realized in anLDD (light doped drain) structure. By realizing the tunnel junctionphoto detector in an LDD structure, it becomes possible to decrease thegeneration of a hot carrier caused by a short channel effect. FIG. 3shows a cross-sectional view of the tunnel junction photo detectorformed in an LDD structure in accordance with an embodiment of thepresent invention.

In FIG. 3, the tunnel junction photo detector 100 is formed on thep-type substrate 100 and includes the source 120 and the drain 130, bothof which are N+ diffusion layers. Here, the source 120 and the drain 130are symmetrical to each other and can have identical device properties.An LDD area 123, which is an n-type area that is lightly doped, isformed in an area that is adjacent to the source 120 and the oxide film141. Moreover, an LDD area 123, which is an n-type area that is lightlydoped, can be formed in an area that is adjacent to the drain 130 andthe oxide film 140. The light-absorbing part 150 can be formed to havethe same length as the distance between the LDD area 123 of the source120 and the LDD area 133 of the drain 130.

When light having greater energy than energy to which doped impuritiesare coupled is irradiated to the light-absorbing part 150, the pluralityof holed become a free state in the light-absorbing part 150, which ispoly-silicon in which p-type impurities are doped, generatingelectron-hole pairs similarly to a reaction occurred electrically in adepletion layer of the photo diode. The generated electron-hole pairsare present in the states of electrons and holes for a predeterminedduration until they are recombined, increasing the number of holeslocally and thus increasing the quantity of electric charge.

The separated electrons freely move outside a grain boundary of thepoly-silicon. Here, if an outside voltage is supplied to the drain 130,the electrons are pulled to near an edge of the LDD area 133 of thedrain. Accordingly, the electrons are accumulated near the edge of thelight-absorbing part 150 that is adjacent to the LDD area 133 andreceive the electric field. The electric field that is relativelystronger is formed as the number of integrated electrons increases.Accordingly, the phenomenon of integration of electrons near the edge ofthe light-absorbing part 150 becomes accelerated. The more intense thelight irradiated to the light-absorbing part 150 is, the moreelectron-hole pairs are formed and the greater electric field is formed.

The tunneling phenomenon occurs readily at a boundary area 141 where thedistance between the LDD areas 123 and the light-absorbing part 150 isthe shortest and at a boundary area 142 where the distance between theLDD areas 133 and the light-absorbing part 150 is the shortest. Atunneling effect occurs while energy level conditions are satisfied inthe boundary areas 141, 142. By the tunneling effect, the electronsintegrated in the boundary areas 141, 142 of the light-absorbing part150 can be moved to the source 120 or the drain 130. In such a case, thetotal quantity of electric charge of the light-absorbing part 150 ischanged. That is, the quantity of electric charge of the holes isincreased by as much as the number of electrons lost by the tunnelingeffect, and the channel 160 is formed between the source 120 and thedrain 130 due to the effect of threshold voltage modulation caused by achange of potential of the light-absorbing part 150. The quantity ofelectric current is increased through the formed channel 160.

Meanwhile, if the intensity of light becomes smaller or the light isblocked, the quantity of electric charge returns to its original state,in an opposite way to the above phenomenon. In case the light isintensely irradiated and then blocked, the light-absorbing part 150becomes to have a quantity of weak (+) electric charge due to theincrease in the quantity of electrons, but an electric field is formedby the electrons accumulated in the boundary area 142 of the LDD area133 of the drain and the boundary area 141 of the LDD area 123 of thesource, in which electric potentials are relatively low. Afterwards, thetunneling effect occurs in the boundary areas 141, 142 in directions theelectrons flowing into the light-absorbing part 150. When the electronsflowed in by the tunneling effect are recombined with the holes, thequantity of (+) electric charge becomes decreased. This will weaken theelectric field by the light-absorbing part 150, and reduce or eliminatethe channel 160 between the source 120 and the drain 130. Accordingly,the electric current flowing through the channel 160 stops flowing.

The channel 160 is designed in a manufacturing process of the tunneljunction photo detector 100 in such a way that the channel 160 is in astate immediately before pinch-off FIG. 4 shows the channel 160 of thepresent invention. In FIG. 4, the channel 160 is generated by a voltagedifference between the source 120 and the drain 130. Moreover, adepletion layer 161 is formed around the source 120, the drain 130 andthe channel 160 due to the supplied voltage. The channel 160 ismanufactured by adjusting a W/L ratio, which is a ratio between itswidth and length, in the manufacturing process so that the channel 160is in the state immediately before pinch-off while no outside voltage issupplied to the source 120 and the drain 130. Here, the W/L can bedesigned experimentally for each manufacturing process of the tunneljunction photo detector since the conditions in which pinch-off occurscan be different for each doping concentration of an element and eachproperty of the tunnel junction photo detector.

The tunneling phenomenon occurs continuously in the boundary area 141,142 between the LDD areas of the source 120 and drain 130 and thelight-absorbing part 150. However, tunneling is more prominent in theside of the drain 130 when the intensity of light is greater, and in theside of the source 120 when the intensity of light is smaller, therebymaintaining the state of equilibrium.

FIG. 6 is a cross-sectional view for illustrating an incident angle oflight of a tunnel junction photo detector in accordance with a firstembodiment of the present invention.

In FIG. 6, the light converged through the micro lens is incident to thelight-absorbing part 150 along a light incident path having apredetermined slope by multiple layers of shades 192. The shades 192 canbe formed by appropriately arranging metal lines for signal transfer anddevice control along the incident path. Formed in between the multiplelayers of shades 192 can be passivation layers 182, which can be formedwith a material that has little reflection of the incident light.

Through the above structure of tunnel junction photo detector, itbecomes possible to flow photoelectric currents that are hundreds tothousands times greater than the conventional photo diode. While theconventional photo diode distinguishes the brightness only by thequantity of electric charge accumulated in the electrostatic capacity,the change in the quantity of electric charge of the light-absorbingpart caused by the light works as the electric field effect in the photodetector in accordance with an embodiment of the present invention,thereby controlling the electric current flow of the channel. Moreover,since the required electric charge can be infinitely supplied throughthe drain, a signal can be self-amplified in the photo detector.Therefore, it is possible to realize a unit pixel in a PPS structure,without introducing an additional signal amplification device. Ofcourse, it is also possible to realize a unit pixel using theconventional APS method. In the present embodiment, however, the unitpixel is realized in the PPS structure using the tunnel junction photodetector, for the convenience of description and understanding.

Hereinafter, some embodiments of a unit pixel of an image sensorrealized using the tunnel junction photo detector in accordance with theabove embodiments will be described with reference to the accompanyingdrawings.

FIG. 7 is a circuit schematic of a unit pixel using the tunnel junctionphoto detector in accordance with the first embodiment of the presentinvention. The unit pixel shown in FIG. 7 includes one tunnel junctionphoto detector 100 and one select transistor 600.

Here, the one select transistor can be formed of various devices, forexample, the conventional MOSFET structure. In this case, the tunneljunction photo detector and the select transistor can be simultaneouslyrealized using a manufacturing process of the conventional MOSFET,simplifying the manufacturing process and saving the cost.

The drain 130 of the tunnel junction photo detector 100 is accessed to apower supply voltage (VDD), and the source 120 is connected to a drain630 of the select transistor 600.

Although the source 120 and drain 130 of the tunnel junction photodetector 100 are symmetrical and identical to each other, thedescription and claims of the present specification will refer the drainas an area accessed to the power supply voltage (VDD) or an outsideelectric charge supply.

The light-absorbing part 150 of the tunnel junction photo detector 100is formed in a floating gate structure that is restricted to allow lightto incident at a gate only. The light-absorbing part 150 does not havemetal silicide formed on an upper surface thereof, and thus it ispossible to absorb the light through the light-absorbing part 150. Ap-type substrate (P-sub), which corresponds to a body of a common NMOSstructure, can be also formed in a floated structure. Therefore, thetunnel junction photo detector 100 is connected electrically with anoutside node through the source 120 and the drain 130.

In the present embodiment, the select transistor 600 can be constitutedwith NMOS. The drain 630 of the select transistor 600 is connected tothe source 120 of the tunnel junction photo detector 100, and a source620 is connected to a unit pixel output terminal (“l_output”). A controlsignal (“Sx”) for the control of on-off of the select transistor 600 canbe supplied through a gate 650.

Moreover, a body 610 of the select transistor 600 can be formed in afloated structure, like the tunnel junction photo detector 100. This isfloating a body 110 of the tunnel junction photo detector 100. In such acase, in the gate control of the select transistor 600 that isswitch-operated, its switching function can be maintained by supplying aslightly higher voltage than the power supply voltage (VDD).

FIG. 8 is a cross-sectional view of a unit pixel constituted with thetunnel junction photo detector and the select transistor in an NMOSstructure in accordance with the first embodiment of the presentinvention.

As illustrated in FIG. 8, both the tunnel junction photo detector 100and the select transistor 600 can be formed in a floated structurehaving the same P-sub as the body. In this case, the source 120 of thetunnel junction photo detector 100 and the drain 630 of the selecttransistor 600 can be formed in a same active area, simplifying thestructure and reducing the size of the unit pixel.

Hereinafter, a tunnel junction photo detector in accordance with asecond embodiment of the present invention will be described.

FIG. 9 is a perspective view of a tunnel junction photo detector 200 inaccordance with another embodiment of the present invention. As shown inFIG. 9, the tunnel junction photo detector 200 includes a V-shapedlight-absorbing part 250. The light-absorbing part 250 is formed above aV-shaped groove that is formed on a p-type substrate 210, and a thinoxide film 240 can be formed in between the light-absorbing part 250 andthe V-shaped groove. The oxide film 240 is formed sufficiently thin sothat a tunneling phenomenon can readily occur. For example, the oxidefilm 240 can be formed in the thickness of 10 nm or less. Thelight-absorbing part 250 can be poly-silicon in which n-type impuritiesare doped.

Like the first embodiment, the light-absorbing part 250 does not have ametallic silicide layer formed on an upper surface thereof but is formedin a floated poly-silicon structure, and functions as an area thatabsorbs light.

The V-shaped groove can be formed by etching the p-type substrate 210.Here, the p-type substrate 210 can be a silicon substrate of which thecrystalline structure is {100}. Said etching process can be ananisotropic etching. The etching process of the {100} type siliconsubstrate is a known art, and thus will not be described herein.

A source 220 and a drain 230 can be formed in respective areas adjacentto both slopes of the V-shaped groove. The source 220 and the drain 230can be formed by injecting high-concentration n-type impurities inrespective locations of the p-type substrate 210. The formed source 220and drain 230 can be separated by the V-shaped groove.

Formed above the source 220 and the drain 230 are metal contacts 221,231 that are respectively connected with outside nodes. The metalcontact 221 of the source 220 is connected with an outside through ametal line 222, and the metal contact 231 of the drain 230 is likewiseconnected with an outside through a metal line 232.

An upper part of the photo detector 200 excluding the light-absorbingpart 250 has a light-blocking layer 280 formed thereon. Thelight-blocking layer 280 blocks the light from being absorbed in areasother than the light-absorbing part 250. A micro lens 270 can be formedabove the light-blocking layer 280 to converge the light incident at thetunnel junction photo detector 200 and guide the incident light to thelight-absorbing part 250.

Referring to FIG. 10, the light converged through the micro lens 270 isincident to the light-absorbing part 250 along a light incident pathhaving a predetermined slope by multiple layers of shades 233. Theshades 233 can be metal lines arranged for signal transfer and devicecontrol. The arrangement of the multiple layers of metal lines can bedesigned in a layout process to provide the incident path having thepredetermined slope. Formed in between the multiple layers of shades 233can be passivation layers 283.

In this case, since the light-absorbing part 250 the tunnel junctionphoto detector 250 shown in FIG. 10 is formed in the V shape, comparedto FIG. 6, the light incident at the predetermined angle through themicro lens 270 be converged to the light-absorbing part 250 moreeffectively.

Moreover, as illustrated in FIG. 11, the V-shaped light-absorbing part250 can re-absorb light reflected by a surface of the light-absorbingpart 250, improving light-absorption efficiency from the flat-typelight-absorbing part of the first embodiment.

Electron-hole pairs are generated inside the light-absorbing part 250 bythe light incident to the light-absorbing part 250. Then, by supplyingan outside voltage to the drain 230, electrons of the generatedelectron-hole pairs are integrated in an area adjacent to the drain 230.Here, if tunneling occurs in the oxide film 240 between the drain 230and the light-absorbing part 250, the electrons integrated in a boundaryarea of the light-absorbing part 150 can be moved to the source 120 orthe drain 130 by the tunneling effect. Accordingly, the quantity ofelectric charge of the holes of the light-absorbing part 250 isrelatively increased by as much as the number of electrons lost by thetunneling effect, and a channel 260 is formed near an edge of theV-shaped groove where the electric field is concentrated.

FIG. 12 shows a tunnel junction photo detector having the V-shapedlight-absorbing part in accordance with another embodiment of thepresent invention. In FIG. 12, a tunnel junction photo detector 300 isformed in a PMOS type. The source 320 and the drain 330 are formed byinjecting high-concentration p-type impurities in areas adjacent to bothslopes of a V-shaped groove formed in a {100} type silicon substrate.The source 320 and the drain 330 are formed in an N-well 315 formed in ap-type substrate 310.

Hereinafter, a tunnel junction photo detector in accordance with a thirdembodiment will be described. FIG. 13 is a perspective view of a tunneljunction photo detector 400 in accordance with a third embodiment of thepresent invention. As illustrated in FIG. 13, the tunnel junction photodetector 400 includes a U-shaped light-absorbing part 450. The U-shapedlight-absorbing part 450 is formed above a U-shaped groove that isformed on a p-type substrate 410. The process for forming the U-shapedgroove is the same as that of the V-shaped groove, but the etching canbe stopped before the complete V shape is made so that the U shapehaving a lower surface not etched is formed.

FIG. 14 shows a tunnel junction photo detector including the U-shapedlight-absorbing part in accordance with yet another embodiment of thepresent invention. In FIG. 14, the tunnel junction photo detector 500 isformed in a PMOS type. The source 520 and the drain 530 are formed byinjecting high-concentration p-type impurities in areas adjacent to bothslopes of a U-shaped groove formed in a {100} type silicon substrate.The source 520 and the drain 530 can be formed in an N-well 515 formedin a p-type substrate 510.

Hitherto, the unit pixel of an image sensor as well as the tunneljunction photo detector of the unit pixel having the technical featuresof the present invention have been described through the aboveembodiments.

Through the above structure, it is possible that the unit pixel of thepresent invention allows hundreds to tens of thousands times greaterphotoelectric currents than the conventional photo diode. This isbecause, unlike the conventional photo diode in which contrast isdistinguished by the quantity of electric charge accumulated in theelectrostatic capacity only, the present invention controls the electriccurrent flow of the source-drain channels owing to the electric fieldeffect from the change in the quantity of electric charge of thefloating gate and at the same time generates an effect ofself-amplification owing to infinite supply of electric charges throughthe drain.

The unit pixel and the tunnel junction photo detector described in theabove embodiments can be realized in a PPS type, which does not need tohave a separate amplification device inside the unit pixel, unlike theconventional CIS.

Moreover, through the above structure, it is possible to realize ahigh-sensitivity/high-speed image sensor.

Since there is little or no parasitic capacitor compared to the outputcurrent of the photo detector inside the pixel of the image sensorhaving the configuration described above, no integration action can bemade until a pixel is selected by a row decoder. Therefore, it becomespossible to develop a high-speed frame image sensor by multi-processinga signal in a modified rolling shutter method.

Since the unit pixel has a very simple structure and is not big, imagesof 500-10,000 fps can be realized by forming a capacitor inside the unitpixel like the global shutter method, storing data simultaneously in ananalog memory and reading the data in high speed.

The above description has been provided in illustrative purposes only,and it shall be appreciated that it is possible for any ordinarilyskilled person in the art to which the present invention pertains toeasily modify the present invention without departing the technicalideas and essential features of the present invention. As used herein,the term “aspect” may be used interchangeably with the term“embodiment.”

Therefore, it shall be appreciated that the embodiments described aboveare illustrative, not restrictive. For instance, any elements describedto be combined can be also embodied by being separated, and likewise,any elements described to be separated can be also embodied by beingcombined.

The scope of the present invention shall be defined not by the abovedescription but rather by the claims appended below, and it shall beunderstood that all possible permutations or modifications that can becontrived from the meanings, scopes and equivalents of the claims areincluded in the scope of the present invention.

What is claimed is:
 1. A photo detector configured to absorb light in aunit pixel of an image sensor transforming the absorbed light to anelectrical signal, comprising: a substrate in which a V-shaped groovehaving a predetermined angle is formed; a light-absorbing part formed ina floated structure above the V-shaped groove, light being incident tothe light-absorbing part; an oxide film formed in between thelight-absorbing part and the V-shaped groove, tunneling occurring in theoxide film; a source formed adjacent to the oxide film on a slope of oneside of the V-shaped groove and separated from the light-absorbing partby the oxide film; a drain formed adjacent to the oxide film on a slopeof the other side of the V-shaped groove and separated from thelight-absorbing part by the oxide film; and a channel interposed betweenthe source and the drain along the V-shaped groove and configured toform flow of an electric current between the source and the drain,wherein the light-absorbing part is doped with first type impurities,and the source and the drain are doped with second type impurities,wherein the light-absorbing part is insulated from the source and thedrain by the oxide film, wherein electron-hole pairs are generated inthe light-absorbing part by the light incident to the light-absorbingpart, tunneling occurs in between at least one of the source and drainand the light-absorbing part by an electric field concentrated in theoxide film, electrons of the electron-hole pairs are moved to at leastone of the source and the drain from the light-absorbing part by thetunneling, and the flow of the electric current of the channel iscontrolled by a change in the quantity of electric charge of thelight-absorbing part caused by the moving of the electrons.
 2. The photodetector of claim 1, wherein the substrate is a {100} type siliconsubstrate, and the V-shaped groove is formed on the substrate byanisotropic etching.
 3. The photo detector of claim 1, wherein thechannel is formed in a state immediately before pinch-off by adjusting aW/L ratio, which is a ratio between a width (W) and a length (L) of thechannel.
 4. The photo detector of claim 1, wherein the source and thedrain are formed by doping the second type impurities in a body, and thebody is floated.
 5. The photo detector of claim 1, wherein a thresholdvoltage of the photo detector is changed due to the tunneling occurredin the oxide film.
 6. The photo detector of claim 1, further comprisinga light-blocking layer formed on a surface other than thelight-absorbing part and configured to block transmission of light inareas other than the light-absorbing part.
 7. A unit pixel of an imagesensor configured to transform absorbed light to an electrical signal,comprising: a photo detector configured to cause an electric current toflow using a change in the quantity of electric charge caused byincident light; and a select device configured to output the electriccurrent generated by the photo detector to a unit pixel output terminal,wherein the photo detector comprises: a substrate in which a V-shapedgroove having a predetermined angle is formed; a light-absorbing partformed in a floated structure above the V-shaped groove, light beingincident to the light-absorbing part; an oxide film formed in betweenthe light-absorbing part and the V-shaped groove, tunneling occurring inthe oxide film; a source formed adjacent to the oxide film on a slope ofone side of the V-shaped groove and separated from the light-absorbingpart by the oxide film; a drain formed adjacent to the oxide film on aslope of the other side of the V-shaped groove and separated from thelight-absorbing part by the oxide film; and a channel interposed betweenthe source and the drain along the V-shaped groove and configured toform flow of an electric current between the source and the drain,wherein the select device comprises: a drain being connected with thesource of the photo detector; a source being accessed to the unit pixeloutput terminal; and a gate configured to receive a control signal froman outside, and a switching operation is performed based on the controlsignal, wherein the light-absorbing part is doped with first typeimpurities, and the source and the drain are doped with second typeimpurities, wherein the light-absorbing part is insulated from thesource and the drain by the oxide film, and wherein electron-hole pairsare generated in the light-absorbing part by the light incident to thelight-absorbing part, tunneling occurs in between at least one of thesource and drain and the light-absorbing part by an electric fieldconcentrated in the oxide film, electrons of the electron-hole pairs aremoved to at least one of the source and the drain from thelight-absorbing part by the tunneling, and the flow of the electriccurrent of the channel is controlled by a change in the quantity ofelectric charge of the light-absorbing part caused by the moving of theelectrons.
 8. The unit pixel of claim 7, wherein the substrate is a{100} type silicon substrate, and the V-shaped groove is formed on thesubstrate by anisotropic etching.
 9. The unit pixel of claim 7, whereinthe source of the photo detector and the drain of the select device areformed in a same active area.
 10. The unit pixel of claim 7, wherein thechannel is formed in a state immediately before pinch-off by adjusting aW/L ratio, which is a ratio between a width (W) and a length (L) of thechannel.
 11. A photo detector configured to absorb light in a unit pixelof an image sensor transforming the absorbed light to an electricalsignal, comprising: a substrate in which a U-shaped groove having apredetermined angle is formed; a light-absorbing part formed in afloated structure above the U-shaped groove, light being incident to thelight-absorbing part; an oxide film formed in between thelight-absorbing part and the U-shaped groove, tunneling occurring in theoxide film; a source formed adjacent to the oxide film on a slope of oneside of the U-shaped groove and separated from the light-absorbing partby the oxide film; a drain formed adjacent to the oxide film on a slopeof the other side of the U-shaped groove and separated from thelight-absorbing part by the oxide film; and a channel interposed betweenthe source and the drain in a lower area of the U-shaped groove andconfigured to form flow of an electric current between the source andthe drain, wherein the light-absorbing part is doped with first typeimpurities, and the source and the drain are doped with second typeimpurities, wherein the light-absorbing part is insulated from thesource and the drain by the oxide film, wherein electron-hole pairs aregenerated in the light-absorbing part by the light incident to thelight-absorbing part, tunneling occurs in between at least one of thesource and drain and the light-absorbing part by an electric fieldconcentrated in the oxide film, electrons of the electron-hole pairs aremoved to at least one of the source and the drain from thelight-absorbing part by the tunneling, and the flow of the electriccurrent of the channel is controlled by a change in the quantity ofelectric charge of the light-absorbing part caused by the moving of theelectrons.
 12. The photo detector of claim 11, wherein the substrate isa {100} type silicon substrate, and the U-shaped groove is formed on thesubstrate by anisotropic etching to have a slope with a predetermineddepth.
 13. The photo detector of claim 11, wherein the channel is formedin a state immediately before pinch-off by adjusting a W/L ratio, whichis a ratio between a width (W) and a length (L) of the channel.
 14. Thephoto detector of claim 11, wherein the source and the drain are formedby doping the second type impurities in a body, and the body is floated.15. The photo detector of claim 11, wherein a threshold voltage of thephoto detector is changed due to the tunneling occurred in the oxidefilm.
 16. The photo detector of claim 11, further comprising alight-blocking layer formed on a surface other than the light-absorbingpart and configured to block transmission of light in areas other thanthe light-absorbing part.
 17. A unit pixel of an image sensor configuredto transform absorbed light to an electrical signal, comprising: a photodetector configured to cause an electric current to flow using a changein the quantity of electric charge caused by incident light; and aselect device configured to output the electric current generated by thephoto detector to a unit pixel output terminal, wherein the photodetector comprises: a substrate in which a U-shaped groove having apredetermined angle is formed; a light-absorbing part formed in afloated structure above the U-shaped groove, light being incident to thelight-absorbing part; an oxide film formed in between thelight-absorbing part and the U-shaped groove, tunneling occurring in theoxide film; a source formed adjacent to the oxide film on a slope of oneside of the U-shaped groove and separated from the light-absorbing partby the oxide film; a drain formed adjacent to the oxide film on a slopeof the other side of the U-shaped groove and separated from thelight-absorbing part by the oxide film; and a channel interposed betweenthe source and the drain in a lower area of the U-shaped groove andconfigured to form flow of an electric current between the source andthe drain, wherein the select device comprises: a drain being connectedwith the source of the photo detector; a source being accessed to theunit pixel output terminal; and a gate configured to receive a controlsignal from an outside, and a switching operation is performed based onthe control signal, wherein the light-absorbing part is doped with firsttype impurities, and the source and the drain are doped with second typeimpurities, wherein the light-absorbing part is insulated from thesource and the drain by the oxide film, and wherein electron-hole pairsare generated in the light-absorbing part by the light incident to thelight-absorbing part, tunneling occurs in between at least one of thesource and drain and the light-absorbing part by an electric fieldconcentrated in the oxide film, electrons of the electron-hole pairs aremoved to at least one of the source and the drain from thelight-absorbing part by the tunneling, and the flow of the electriccurrent of the channel is controlled by a change in the quantity ofelectric charge of the light-absorbing part caused by the moving of theelectrons.
 18. The unit pixel of claim 17, wherein the source of thephoto detector and the drain of the select device are formed in a sameactive area.
 19. The unit pixel of claim 17, wherein the substrate is a{100} type silicon substrate, and the U-shaped groove is formed on thesubstrate by anisotropic etching.
 20. The unit pixel of claim 17,wherein the channel is formed in a state immediately before pinch-off byadjusting a W/L ratio, which is a ratio between a width (W) and a length(L) of the channel.